Well Al-Queda say they didnt do it, which is a bit strange, one would think they'd be quite happy to take credit for such a blow against symbols of freedom and democracy, which they hate so much. Kinda strange too that the hijackers would hide their identitys, when they believed they were doing Gods work and would go straight to heaven and collect 77 virgins for their actions.
As for evidence it was premeditated, arabic flight training manuals, a copy of the Koran, pilot uniforms and even names of the other suspects were found. Mohammed Attas passport miraculously survived the first plane crash, and copys of his 4 page suicide note was found at the site of the Pennsylvania crash. 2 other copys were found. Abdulaziz Al-Omari and Mohammed Atta were caught on security cameras twice in Portland on their way to Boston. Despite the fact that Al-Omari was in Saudi at the time.